Microelectromechanical (MEM) devices can be formed by bulk or surface micromachining processes. In bulk micromachining, a monocrystalline substrate is photolithographically patterned and etched to form a particular MEM device. In surface micromachining, a plurality of layers of polycrystalline silicon (also termed polysilicon) and a sacrificial material such as silicon dioxide or a silicate glass are alternately deposited and photolithographically patterned to form a MEM device. Up to three or four layers or more of structural polysilicon can be used to form surface micromachined MEM devices which can include numerous interconnected moveable elements (e.g. gears, wheels, carriages, linkages, hinges, ratchet pawls etc.,) with a plurality of contacting surfaces that are subject to wear upon repeated operation.
Wear has been identified as a significant failure mechanism and reliability issue for MEM devices, especially for load-bearing surfaces. One prior approach for reducing wear in MEM devices is based on the deposition of a thin layer of a low-friction polymeric coating on contacting surfaces of MEM devices (e.g. a polytetrafluoroethylene coating deposited by plasma-enhanced chemical vapor deposition, or a polymer deposited from a wet chemical solution). Such polymeric coatings are relatively soft so that wear is reduced simply by decreasing the coefficient of friction between the coated contacting surfaces. The long-term behavior of MEM devices having low-friction polymeric coatings is presently unknown, especially when the MEM devices are operated in vacuum environments (e.g. in space or in an evacuated package).
Another approach for reducing wear in MEM devices is based on the substitution of diamond or silicon carbide for polysilicon in order to provide an intrinsically hard material for forming the MEM devices. This approach is disadvantageous in that it deviates from conventional integrated circuit (IC) processing technology and tool sets which have traditionally been leveraged for use in surface micromachining and which allow integration of conventional integrated circuitry with MEM devices, sensors, optical devices etc., on the same substrate with no assembly required. Additionally, the use of diamond or silicon carbide for forming MEM devices can be problematic for process integration with ICs formed on the same substrate as the MEM devices.
What is needed is a hard coating for contacting or rubbing surfaces in MEM devices that is compatible with conventional IC process technology in terms of materials, film deposition equipment and process integration techniques.
An advantage of the present invention is that a thin conformal coating of tungsten can be formed over one or more semiconductor surfaces of a MEM device in a manner that is compatible with conventional IC process technology.
A further advantage of the present invention is that the conformal tungsten coating can be formed over the semiconductor surfaces within a MEM device after an etch release step whereby a sacrificial material used to surrounding elements of the MEM device during build-up of the device structure is removed, at least in part. The conformal tungsten coating can be formed on all exposed semiconductor surfaces of the released elements in a single process step. Furthermore, the tungsten coating can penetrate laterally between a pair of closely-spaced or contacting surfaces to coat those surfaces comprising the semiconductor material, while not coating other surfaces formed from non-semiconductor materials (e.g. dielectric materials such as silicon dioxide, silicon nitride or silicate glasses).
Another advantage of the present invention is that the ability of the conformal tungsten coating to be formed between contacting surfaces in a MEM device can be used to free elements of the MEM device suffering from stiction (i.e. unintentional adhesion to adjacent surfaces) after an etch release process step.
Yet another advantage of the present invention is that the conformal tungsten coating can be formed over semiconductor surfaces of an element of a MEM device without increasing the overall dimensions of that element.
A further advantage of the present invention is that the conformal tungsten coating provides a substantial improvement in the wear resistance and reliability of a MEM device.
Still another advantage of the present invention is that the conformal tungsten coating can be formed to a predetermined thickness of generally 5-50 nanometers by a self-limiting process which automatically terminates to prevent the formation of an excessively-thick tungsten coating.
A further advantage of the present invention is that the conformal tungsten coating can be formed at a temperature below about 600.degree. C. for compatibility with standard integrated circuit (IC) processes, including the use of aluminum or an aluminum alloy as an interconnect metallization.
Yet another advantage of the present invention is that the conformal tungsten coating can improve the electrical conductivity or optical reflectivity of semiconductor elements within a MEM device.
Still another advantage of the present invention is that a wear-resistant tungsten coating can be formed on many different types of Group IV semiconductor surfaces including silicon, germanium, silicon-germanium alloys and silicon carbide. The present invention is further applicable to forming a tungsten coating on III-V semiconductors such as gallium arsenide.
These and other advantages of the present invention will become evident to those skilled in the art.